The airline has a large fleet of A330 aircraft and the majority of these are relatively young although 40% need to be replaced as soon as is possible as they are between 16 and 22 y.o.
Cathay have already indicated that they are in the market for a further 25 (about) wide body workhorse mid flight range aircraft with high freight capacity. This precludes any change to narrow bodies on the A330 routes.
Although Boeing desperately needs business for the B787 and will deal on cost and payment it is likely in my opinion that Cathay will go again with the A330 as it ticks most of the boxes.